export let Base64 = {  // 转码表  
    tables : [      
        'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H',      
        'I', 'J', 'K', 'L', 'M', 'N', 'O' ,'P',      
        'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X',      
        'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f',      
        'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n',      
        'o', 'p', 'q', 'r', 's', 't', 'u', 'v',      
        'w', 'x', 'y', 'z', '0', '1', '2', '3',      
        '4', '5', '6', '7', '8', '9', '+', '/'  
    ],  
    UTF16ToUTF8 : function (str) {    
        let results = [], len = str.length;    
        for (let i = 0; i < len; i++) {      
            let code = str.charCodeAt(i);      
            if (code > 0x0000 && code <= 0x007F) {        /* 一字节，不考虑0x0000，因为是空字节           U+00000000 – U+0000007F   0xxxxxxx        */        
                results.push(str.charAt(i));      
            } else if (code >= 0x0080 && code <= 0x07FF) {        /* 二字节           U+00000080 – U+000007FF   110xxxxx 10xxxxxx           110xxxxx        */        
                let byte1 = 0xC0 | ((code >> 6) & 0x1F);        // 10xxxxxx        
                let byte2 = 0x80 | (code & 0x3F);        
                results.push(String.fromCharCode(byte1),String.fromCharCode(byte2));
            } else if (code >= 0x0800 && code <= 0xFFFF) {        /* 三字节           U+00000800 – U+0000FFFF   1110xxxx 10xxxxxx 10xxxxxx           1110xxxx        */                      let byte1 = 0xE0 | ((code >> 12) & 0x0F);        // 10xxxxxx       
                let byte2 = 0x80 | ((code >> 6) & 0x3F);        // 10xxxxxx        
                let byte3 = 0x80 | (code & 0x3F);        
                results.push(String.fromCharCode(byte1), String.fromCharCode(byte2),  String.fromCharCode(byte3));      
            } else if (
                code >= 0x00010000 && code <= 0x001FFFFF) {        // 四字节        // U+00010000 – U+001FFFFF   11110xxx 10xxxxxx 10xxxxxx 10xxxxxx      
            } else if (
                code >= 0x00200000 && code <= 0x03FFFFFF) {        // 五字节        // U+00200000 – U+03FFFFFF   111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx      
            } else /** if (code >= 0x04000000 && code <= 0x7FFFFFFF)*/ {        // 六字节        // U+04000000 – U+7FFFFFFF   1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx      
            }    
        }    
        return results.join('');  
    },  
    UTF8ToUTF16 : function (str) {   
        let results = [], len = str.length;    
        let i = 0;    
        for (let i = 0; i < len; i++) {      
            let code = str.charCodeAt(i);      // 第一字节判断      
            if (((code >> 7) & 0xFF) == 0x0) {        // 一字节        // 0xxxxxxx     
                results.push(str.charAt(i));      
            } else if (((code >> 5) & 0xFF) == 0x6) {        // 二字节        // 110xxxxx 10xxxxxx        
                let code2 = str.charCodeAt(++i);        
                let byte1 = (code & 0x1F) << 6;        
                let byte2 = code2 & 0x3F;        
                let utf16 = byte1 | byte2;        
                results.push(String.fromCharCode(utf16));      
            } else if (((code >> 4) & 0xFF) == 0xE) {        // 三字节        // 1110xxxx 10xxxxxx 10xxxxxx        
                let code2 = str.charCodeAt(++i);        
                let code3 = str.charCodeAt(++i);        
                let byte1 = (code << 4) | ((code2 >> 2) & 0x0F);        
                let byte2 = ((code2 & 0x03) << 6) | (code3 & 0x3F);        
                let utf16 = ((byte1 & 0x00FF) << 8) | byte2        
                results.push(String.fromCharCode(utf16));      
            } else if (((code >> 3) & 0xFF) == 0x1E) {        // 四字节        // 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx      
            } else if (((code >> 2) & 0xFF) == 0x3E) {        // 五字节        // 111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx      
            } else /** if (((code >> 1) & 0xFF) == 0x7E)*/ {        // 六字节        // 1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx      
            }    
        }    
        return results.join('');  
    },  
    encode : function (str) {    
        if (!str) {      
            return '';    
        }    
        let utf8    = this.UTF16ToUTF8(str); // 转成UTF-8    
        let i = 0; // 遍历索引    
        let len = utf8.length;    
        let results = [];    
        while (i < len) {      
            let c1 = utf8.charCodeAt(i++) & 0xFF;      
            results.push(this.tables[c1 >> 2]);      // 补2个=      
            if (i == len) {        
                results.push(this.tables[(c1 & 0x3) << 4]);        
                results.push('==');        
                break;      
            }      
            let c2 = utf8.charCodeAt(i++);      // 补1个=      
            if (i == len) {
                results.push(this.tables[((c1 & 0x3) << 4) | ((c2 >> 4) & 0x0F)]);
                results.push(this.tables[(c2 & 0x0F) << 2]);        
                results.push('=');        
                break;      
            }       
            let c3 = utf8.charCodeAt(i++);      
            results.push(this.tables[((c1 & 0x3) << 4) | ((c2 >> 4) & 0x0F)]);                    results.push(this.tables[((c2 & 0x0F) << 2) | ((c3 & 0xC0) >> 6)]);                   results.push(this.tables[c3 & 0x3F]);    
        }    
        return results.join('');  
    },  
    decode : function (str) {    //判断是否为空    
        if (!str) {      
            return '';   
        }    
        let len = str.length;    
        let i   = 0;    
        let results = [];    //循环解出字符数组    
        while (i < len) {      
            let code1 = this.tables.indexOf(str.charAt(i++));      
            let code2 = this.tables.indexOf(str.charAt(i++));      
            let code3 = this.tables.indexOf(str.charAt(i++));      
            let code4 = this.tables.indexOf(str.charAt(i++));      
            let c1 = (code1 << 2) | (code2 >> 4);      
            results.push(String.fromCharCode(c1));      
            if (code3 != -1) {        
                let c2 = ((code2 & 0xF) << 4) | (code3 >> 2);        
                results.push(String.fromCharCode(c2));      
            }      
            if (code4 != -1) {        
                let c3 = ((code3 & 0x3) << 6) | code4;        
                results.push(String.fromCharCode(c3));      
            }    
        }    
        return this.UTF8ToUTF16(results.join(''));  
    }
};